Polymer powders have been used for manufacturing articles by rotational molding, a process that is well known in the art. The powder to be rotationally molded melts against the hot mold wall and forms a layer thereagainst which will eventually be the outer surface of the molded object. The thus formed surface of the molded object may consist of a single layer, but may also consist of a number of layers. Foam structures between or against either a layer or layers may be applied to improve the insulating properties or to limit the weight of the objects. The surface layer and the foam laying thereagainst must adhere to each other. Now, this adhesion leaves much to be desired in the processing of polyethylene. The same is true with polypropylene. This is an acute problem in producing surf-boards or wind surf boards, made for the most part, by the rotational molding process wherein a wall of polyethylene, particularly of polyethylene having a high or medium density of at least 0.930 g/ml forms the outer surface, i.e. envelopes, the core of such boards. Polyethylene having a lower density may also be processed for this purpose. Generally speaking polyurethane foam forms the core of such boards.
In use however, the polymer wall may be damaged and water may penetrate into the foam. Damage, will occur less rapidly when there is good adhesion between the wall and foam. In such circumstances accompanied by outer wall damage, water can then penetrate only into the foam in an area immediately under the damaged portion of the wall. If the wall-core adhesion is poor, then water may spread between the wall and foam and thus be absorbed throughout the object. Good adhesion is thus highly desirable for in particular wind surf boards as well as for other objects built up from similar components. There is, therefore, a widespread need for polymer compositions to satisfy this objective.
There are other uses of hollow article made by rotational molding. So without filling the hollow article with a foam the article may be used as e.g. a container for organic or other liquids. It may be required that the inner layer has mechanical, physical or other properties that differ from the properties of the outer layer. Such a condition may also be imposed when the requirements for the wall of the hollow article cannot be met by one material. One can fulfill such conditions by making the hollow article from two or more layers constituting the wall of the hollow article. It has been disclosed to produce such articles in a two shot rotational molding process, molding first an outer layer, cooling the rotational mold, adding into the mold a second charge of polymer powder and molding an inner layer. The first and second charge can consist of different materials, but they can consist also of the same or closely related materials having different properties. So in order to make surf-boards one can make in a first shot an outer layer of stabilized polyethylene that releases easily from the mold and has good wheathering properties, whereas in a second shot an inner layer of the same or another grade of polyethylene, that has not been stabilized is produced. Due to the lack of stabilization said inner layer becomes somewhat oxidized during rotational molding and said oxidized polyethylene adheres well to a core consisting of polyurethanfoam. Such two shot processes are laborious and costly.
The compositions described in U.S. Pat. No. 4,307,133 for coating metal pipes consisting of a polyolefin powder composition having from 0.5% to 80% by weight of unstabilized polyethylene powder and from 20% to 99.5% by weight of a stabilized polyolefin powder are not suitable for producing hollow articles that are to be filled with e.g. a polyurethan foam. Such compositions adhere strongly to the metal object while the exterior of the coating exhibits some resistance to oxidation and other weathering influences. When such compositions are rotationally molded and filled subsequently with e.g. polyurethan foam they adhere strongly to the mold and mold release is very difficult or not possible at all whereas the adherance to the foamed core is unsatisfactory.
Improved adhesion of polyethylene to metal substrates by mixing the polyethylene with oxidized polyethylene is proposed in U.S. Pat. No. 3,639,189. The oxidized polyethylene is obtained by heating polyethylene in an oxygen atmosphere at temperatures that may range from 90.degree. C. to the crystalline melting point of the polyethylene until the desired degree of oxidation has been reached. The un-oxidized polyethylene is then mixed in the melt with the oxidized polyethylene and granulated. Subsequently the granulate is used as a starting material applying polymer layers to metal substrates. This proposal suffers from several drawbacks. The oxidation of polyethylene is laborious and significantly increases the cost of the composition used as starting material. In applications as for instance, manufacturing windsurf boards by rotational molding such a composition is not suitable, because the oxidized polyethylene reduces the resistance of the polyethylene composition against atmospheric influences and degradation. The decrease of the wheathering resistance is highly undesirable for an outer wall, such as the outer skin of a windsurf board, which is exposed to atmospheric influences in an outdoor environment.
Now, it is true that this disadvantage might be combated by incorporating more stabilizing agents in the polyethylene composition. However, incorporating more such agents is just not practical since the product costs will significantly and undesirably escalate. Using compositions containing oxidized polyethylene to produce objects in a die or mold by, for instance rotational molding, has the additional disadvantage that such compositions also adhere to the mold. If non-oxidized polyethylene is used, then release agents are a necessity if the polyethylene enveloped article is to be released from the mold. Besides, despite the use of release agents difficulties will still occur in attempting to remove an oxidized-polyethylene-coated-article from a mold.
Further attempts to solve these drawbacks include polyolefin compositions of stabilized and unstabilized polyethylenes wherein the focus is on controlling the crystalline melting points of the stabilized and unstabilized components. Such compositions do provide some improved adhesion to a substrate and improved release from the wall of the mold. However, imperfections are not infrequent. For instance, the adhesion of the polyolefin wall to a foamed core is sometimes unsatisfactory and the release from the mold is often very difficult. It has now been found that the results are dependent upon the processing conditions. It is supposed that the degree of separation of the stabilized and of the unstabilized polyolefin depends on the method and rate of heating of the mold and on other processing conditions.